Contradiction in GPL Licensing FAQ
Can my organization use Blender internally without giving up our valuable changes to our competitors?


The question starts with Can..? and indeed it's possible. All your coworkers and people who come in contact would have to willingly not distribute it any further and to your competitors.

If anyone decides that he wants the source code and he wants to either build upon it or further distribute it (even to your competition), he has all the rights to do it, because the software is licenced GPL.